The present invention relates to a support used for the lithographic printing plate, which is formed by providing an anodic oxide film on the roughened surface of aluminum alloy plate and further coating thereon a photosensitive material. The support for a lithographic printing plate gives an uniform rough surface by electrochemical roughening treatment and is excellent in the strength, thermal softening-resistant characteristic and printability.
Conventionally, as the lithographic printing plates, those coated with the photosensitive material onto the aluminum plate provided the surface treatments such as roughening treatment, treatment of forming anodic oxide film, etc. are used. Among these, one used most widely is so-called PS plate coated with the photo-sensitive material beforehand and ready to be printed instantaneously. To such lithographic printing plate, the platemaking treatments such as exposure to light for imaging, development, washing with water, lacquering, etc. are given to obtain the press plate. It is a well-known fact that development of an exposed plate results in an image portion which is composed of insoluble photosensitive material and a non-image portion which results from the removal of soluble photosensitive material from the exposed plate, thereby exposing the underlying aluminum surface. The exposed aluminum layer, i.e., the non-image area, because it is hydrophilic, is the water attracting area.
As the supporter for such lithographic printing plate, aluminum plate which is light in weight and excellent in the surfaceprocessibility, workability and corrosion resistance is used, in general, and, as the conventional materials offered to this purpose, there are aluminum alloys with a thickness of 0.1 to 0.8 mm such as JIS 1050 (pure Al with a purity of not less than 99.5 wt. %), JIS 1100 (alloy consisting of Al--0.05 to 0.20 wt.% Cu), JIS 3003 (alloy consisting of Al--0.05 to 0.20 wt. % Cu1.5 wt. % Mn). etc. The surface of these materials is roughened by the roughening methods through the processes employed either one or not less than two of mechanical method, chemical method and electrochemical method and thereafter the anodic oxidation treatment is given preferably.
Concretely, an aluminum lithographic printing plate described in Japanese Unexamined Patent Publication No. sho 48-49501 wherein the mechanical roughening treatment, chemical etching treatment and treatment of forming anodic oxide film are given in this order, an aluminum lithographic printing plate described in Japanese Unexamined Patent Publication No. sho 51-61304 wherein the chemical etching treatment and treatment of forming anodic oxide film are given in this order, an aluminum lithographic printing plate described in Japanese Patent Publication No. sho 54-146234 wherein the electrochemical treatment, post-treatment and treatment of forming anodic oxide film are given, an aluminum lithographic printing plate described in Japanese Patent Publication No. sho 48-28123 wherein the electrochemical treatment, chemical etching treatment and treatment of forming anodic oxide film are given in this order, an aluminum lithographic printing plate described in Japanese Unexamined Patent Publication No. sho 54-63902 wherein the mechanical roughening treatment, chemical etching treatment and electrochemical roughening treatment are given in this order, and the like are known. By selecting the photosensitive layer to be coated onto such support appropriately, it is possible to obtain distinct prints amounting to even a hundred thousand sheets.
However, there is a request for obtaining more sheets of prints from a printing plate (improvement in the printing tolerance). In such case, a method is effective wherein, after the exposure to light and the development treatment of PS plate having the aluminum alloy plate as support by usual method, the heating treatment (so-called burning treatment) is made at high temperature to reinforce the image area, and this method is described in detail in Japanese Patent Publication No. sho44-27243 and sho 44-27244. Although the heating temperature and the time of such burning treatment depend upon the type of resins forming the image, a range of 200.degree. to 280.degree. C. and that of 3 to 7 minutes were common.
Recently, with respect to the burning treatment, higher temperature and shorter time have been desired from the reasons of the improvement in printing tolerance and shortening of time for burning treatment. However, with the aluminum alloy plates having been used conventionally, the recrystallization phenomenon of aluminum occurs when heating at a high temperature of more than 280.degree. C., and, because of extreme lowering of the strength and loss of the stiffness of plate, the handling of plate becomes very difficult resulting in the shortcomings such that the setting of plate on the press becomes impossible, that the registering of color on plate cannot be made in multicolor printing, and the like. Therefore, the stable aluminum alloy plate rich in heat resistance is desired.
On the other hand, in these days when the printing speed has been increased accompanying with the progress in the printing technology, the stress exerted on the printing plate secured mechanically to both ends of plate cylinder in the press is increased. Consequently, when the tensile strength is deficient, these secured portions are deformed or damaged to cause the obstructions such as discrepancies in print etc. and, when the fatigue strength is deficient, the plate is broken due to the repeated stresses exerting on the folded portions of printing plate (breakdown by clamping) resulting in the impossibility in printing frequently.
With conventional aluminum alloy plate according to JIS 1050, uniform rough surface and proper surface roughness can be obtained by the electrochemical roughening treatment and the staining of nonimage area is hard to occur during printing, but the strength and the thermal softening-resistant characteristic are poor. Moreover, conventional aluminum alloy plate according to JIS 3003 has more excellent strength and thermal softening-resistant characteristics, but uniform rough surface and proper surface roughness cannot be obtained and further there is a shortcoming that the staining of nonimage area is also apt to occur during printing. Furthermore, the thermal softening resistance has also become not always to be said that it is sufficient for the severe level of request in recent years.
Accordingly, the purpose of the invention is to provide a support for lithographic printing plate which has the strength (concretely, tensile strength and fatigue strength) and the thermal softening-resistant characteristic and which enables to give uniform rough surface and proper surface roughness by the roughening treatment, in particular, electrochemical roughening treatment resulting in that the staining of the nonimage area is hard to occur during printing.